 Now, since we are talking about elemental semiconductors, so we will see whether I can further classify elemental semiconductors because earlier what we have done, we have classified the semiconductor itself. We have classified the type of semiconductor which are intrinsic semiconductor, extrinsic semiconductor, we have say elemental and compound semiconductor. So compound semiconductor is not something which we are studying in this particular class. So since we are discussing about the elemental semiconductor, let us further classify elemental semiconductor and see whether there are different kinds of elemental semiconductor also. So there are two kinds of elemental semiconductors. The first one is intrinsic and the second one is extrinsic. So when I say intrinsic, what I mean to say a semiconductor which is in purest form, let us say it is germanium semiconductor. So entire semiconductor is just made up of a single element germanium. What I mean to say when I talk about extrinsic semiconductor is there will be small amount of impurity added to it. For example, arsenic may be added inside the germanium or silicon semiconductor. So these impurity will tremendously increase the conductivity of the semiconductor. That we will discuss little bit later. Right now let us talk about intrinsic semiconductor and see how it works. So we are trying to gain more insight with respect to how it works and what happens inside the crystal lattice or how the electron comes out of a particular nucleus and things like that. So since we are taking example of silicon and germanium only, these are group 4 elements. We know that the nucleus will have plus 4 charge. So let us say we have silicon and germanium lattice, it will be like this. All of you draw this. So every circle represents a nucleus. And these branches which I am drawing, these branches represent some sort of bond. It is a covalent bond, electron sharing is happening. So one line represents one electron is plus 4, plus 4, plus 4. So we have one electron here, one electron there. So like this electron sharing is happening and silicon or germanium complete their octets. So you can see that electrons are tightly held inside the covalent bond. And these electrons cannot just break free on their own. So what will happen is that if you supply energy, now you can supply energy using different means. You can supply energy using EM wave, you can just give heat, you can heat it up or you can apply a large amount of electric field. So there are multiple ways you can generate energy and give it to the electron. So once let us say this electron gets sufficient energy, what will happen? This electron will just leave the bond. So bond is sort of broken. So there will be a site which is open here and electron can come over here and set. So this electron which was there earlier at this site becomes free and this electron can come under the influence of external electric field and can conduct electricity. At the same time this hole, you can say that this hole is positively charged which has plus E because earlier it was neutral. So if minus E charge comes out, plus E will remain. So this hole will also attract the neighboring electrons. So this electron suppose jumps over there like that. So hole will get created here. So in a way hole is also moving, getting it. So hole also moves and electron also move. So in the intrinsic semiconductor there is a current due to electron movement and current due to hole movement. So the total current is due to both of them. And because of the charge conservation, I can say that the amount of electron current will be equal to amount of hole current. Because number of holes that are getting generated per unit time is same as number of free electron getting generated per unit time. So the rate of generation should be same. So like this you can understand that how an intrinsic semiconductor works. Any doubts both of you? Anything? No sir. No. Okay tell me one thing, why carbon is not a semiconductor? Carbon also has the same crystal lattice. Why carbon is not a semiconductor? But silicon and germanium are the semiconductors. So do you like need a lot of energy to remove the electron? Why? See you are not too sure. So see the thing is when it comes to carbon, the outer most shell electron, they are forming the bonds and they can only become free and start conducting. The inner shell electron will not come out. But when you compare the outer most shell electron of carbon with outer most shell electron of silicon and germanium, carbon is much closer to the nucleus, the outer most shell. So because of that you need lot of energy and the energy becomes more than 3 electron gold. So carbon is classified as non-metal and not the semiconductors. So this is a brief of intrinsic semiconductor. So we do not use intrinsic semiconductor. All the useful application happens for the extrinsic semiconductor only. So we can discuss extrinsic semiconductor in greater detail now. Now extrinsic semiconductor like what we discussed earlier, you can put impurity in extrinsic semiconductor. All of you write down extrinsic semiconductor. So it is nothing but you are adding impurity to the intrinsic semiconductor. So it is intrinsic plus a small amount of impurity. Now impurity does not mean that you can add anything. So although the name is impurity but it is very selective kind of material that you have to add. So there are two ways you can add impurity or two types of impurity which you can add in intrinsic semiconductor and make it extrinsic. These are you can add trivalent, if you add trivalent impurity on intrinsic, this will become p-type semiconductor. And if you add pentavalent, you add pentavalent impurity to the intrinsic. This will become n-type semiconductor. So these are the two types of extrinsic semiconductor that we are going to discuss. Let's see first what happens if I add pentavalent impurity to a pure semiconductor. Let's see how it affects the overall conductivity. So we will first talk about n-type semiconductor. So arsenic is one of the example of n-type semiconductor. So the amount of impurity that you add is very less. It is like 1 out of 1 million atoms. So like this you add the impurity. But then even though it is just 1 out of 1 million, there are so many atoms that even this kind of concentration of the impurity can also generate good amount of free electrons that we are requiring for them to conduct the electricity. Let's see how it works. Suppose you have impurity which is pentavalent. Now of course since it is very less, 1 out of million sort, so a pentavalent nucleus is bigger chance that it will be surrounded by the tetravalent elements. The impurity will be surrounded by the actual atoms because they are much more in numbers. So now tell me how many valence electron a pentavalent element will have? So tell me how many valence electron the pentavalent element will have? You are not able to hear me? So they will have 5. You have problem in answering some questions? Kondaneya, why I mean you can't answer the simple question which I am asking here. Let's keep it interactive. Let's not keep it like a monologue. Keep answering whenever I am asking. Otherwise you can as well watch a recorded lecture what is the point of taking a live session, isn't it? Okay sir. Anyways, so pentavalent element will have 5 valence electrons. So the 4 electron will get shared by the 4 neighboring tetravalent elements. What will happen to the 5th valence electron? Will it create a bond? Not too sure. It will not create a bond. So it will remain in the orbit of pentavalent element. Only 4 electron can create a bond. So this electron is not participating in the bond. Now tell me whose energy will be more? This electron which is not participating in the bond or the electron that are participating in the bonds? This one, right? Electron if it participate in a bond its energy will go down. So if you draw an energy diagram for this n-type semiconductor, let's say this is energy, right? This was the normal so to say valence band and this is the normal conduction band. This one. This is conduction band, the upper one and the lower one is the valence band. Now can you show the energies of the electron? All of you, there will be electrons like this. What are these electrons? These electrons have participated in the bond. These valence electrons have participated in formation of bonds. Where the energy level of these electrons will be which are not participating in the bond formation? In the higher one. Will it be in the conduction band? Inside the conduction band? This is conduction band. See now if electron is in the conduction band it means that the electron is free to jump from one nucleus to other. Is this electron free to jump from one nucleus to other? It is not. No sir. It is not. But then its energy is higher than the valence electron but is not yet in the conduction band. So there will be a layer at this location. But then the good thing about this particular electron level is that it requires very very less amount of energy for these electrons to jump to conduction band and start conducting. Getting it? So you do not need to now put the valence electron in the conduction band because you have these electrons which already have higher energy. So you just need to supply this much energy. You know this energy is approximately 0.01 electron volt. So we have increased the conductivity of this material by doping it or putting impurity of pentavalent element. All of you clear about it? So but for it to go to the conduction band it should enter like the bonds. So for that one electron should be removed and for that you need energy so that is more than 0.01 electron volts. No this electron you are not just removing electron from one of the atom. See if you remove this electron from here some of the electron will get attracted and come here. What you are doing is you are just making it free. It is loosely held to this nucleus. It will go to some other nucleus and some other electron will come back here at plus 5 site. So like that you are just increasing the mobility of the electron. You are not creating a charge as such. Or you are not taking electron outside of the material. It remains inside only. But you are increasing its movement. So this is n type semiconductor. Now let's see what is so special about the p type semiconductor. So in p type semiconductor we are doping a trivalent element on tetravalent element. So let us say I am doping boron or aluminum. This is plus 3. So what will happen here is that plus 3 has only 3 valence electrons. So it will create boron like this and it will have an incomplete octet. So there will be a hole like that. Getting it both of you? Yes sir. Now this hole will try to attract the electron. So electron will leave one of the site and go to the other site. And this will move the hole here. So like that movement of the charged particles will start and you can say that electricity is not conducted. Getting it that's how the p type semiconductor works. And let us try to build the energy diagram of p types. So let's say this is the normal valence band and we can say that this is normal conduction band. These are the electrons that have participated in the bond formation. Now can you guess how we can draw the energy levels further? So technically you don't need like any energy to like move it right because anyways there will be a hole. So by itself the electrons will start moving. Okay now tell me one thing. If there is a site the energy will be there with respect to the site also. So this hole which is there that hole itself will have some energy. And when electron is inside that hole then it will have a certain energy. Whose energy will be more a site without or a hole without electron or when electron reaches the hole. Which one the energy is more? The when the electron reaches the hole it completes the octet. So energy of the hole goes down. So the hole has slightly higher energy if it doesn't have an electron. So we can say that this is the level for the holes. Now for the electron to start conducting all electron have to do is to jump from the valence band to where? Yes sir. No they have to just jump to the hole. That's what it is happening right? It leaves this electron, leaves this site and jumps onto the hole and electricity gets conducted. So it doesn't need to go to conduction band to conduct electricity. It can just reach up to the site of the hole and electricity will get conducted. So even this energy is 0.01 electron volt. Are you guys clear? Yes sir. And one more thing there is a thermal equilibrium condition. The derivation is not in our syllabus but then the formula is given. So we will just take it as it is. Write down thermal equilibrium condition. This tells us that number of electrons per unit volume multiplied by number of holes per unit volume is equal to n i square. So you understand right? Any n h are the electron and hole concentration in a particular type of semiconductor. And n i is the concentration of charge carriers in the intrinsic semiconductor. Suppose I take silicon semiconductor so it will have a fixed value of n i. For silicon it has a fixed value of number of charge carriers per unit volume. For germanium fixed value of n i. But when you add impurity inside it and number of electrons and number of holes they will get changed. But the product of that will remain a constant which is n i square. Yes sir. Fine. Let us take a numerical. Both of you do this. Suppose the pure silicon crystal has 5 into 10 raise power 28 atoms per meter cube. Pure silicon has this much concentration of atoms. It is doped by one parts per million of pentavalent arsenic. You need to calculate number of electrons and number of holes. What is given is n i equal to 1.5 into 10 raise power 16 per meter cube. Do this. What are we done? Sir is n e 5 into 10 to the power 22. Yes. And n h will be 4.5 into 10 to the power 9. You got it? Yes sir. Okay. Good. So basically one part per million is one out of 10 to the power 6. So if you are putting a doping of arsenic like that. So you can count number of arsenic atoms per unit volume. That is nothing but number of free electron that can get generated. Because every arsenic atom will give you one free electron. So n e will be nothing but number of arsenic atoms per unit volume. This divided by 10 raise power 6. Now we have n i and n e and n e into n h is equal to n i square. From there you can get the value of n h. So numericers like these can get asked in your exams.